As the most abundant plasma protein, serum albumin has emerged as a versatile carrier for therapeutic agents, primarily for treating diabetes, cancer, rheumatoid arthritis and infectious diseases (Elsadek B et al., Kratz F. J Control Release 2012; 157(1): 4-28). Serum albumin was also used directly as an imaging probe after labeling with fluorescent dyes for optical imaging (Klohs J. et al., J Neurosci Methods. 2009; 180(1): 126-132), radioisotopes for scintillation scanning or positron emission tomography (PET) (McAfee J G et al., J Nucl Med 1964; 5:936-946; Hoffend J. et al., Nucl Med Biol 2005; 32(3): 287-292), or Gd3+ for magnetic resonance imaging (MRI) (Lauffer R B et al., Radiology. 1998; 207(2): 529-538). The major applications of labeled serum albumin mentioned above include blood pool imaging and angiography.
In clinical nuclear medicine, kit preparations for indirect and direct 99mTc-radiolabeling of red blood cells (RBCs) are still the dominant methods for blood pool imaging. Compared with single-photon emission computed tomography (SPECT), PET is more sensitive and has higher spatial resolution with clinical instruments. However, to date, only very few blood-pool tracers have been introduced for PET. For example, carbon monoxide (CO) containing either 11C or 15O has been used to label RBCs for PET. However, due to their short half-lives (20.4 min for 11C and 2.05 min for 15O), these tracers can only be used in centers with an in-house cyclotron. Moreover, the gaseous form of CO and the need for administration by inhalation necessitates sophisticated equipment for either human or animal studies.
Commercial availability of the species specific isoforms of albumin including human serum albumin (HSA) makes blood drawing unnecessary. In fact, 131I-labeled HSA is the only FDA approved radiologic agent for measuring blood volume. For imaging purpose, albumin has been labeled with various radioisotopes for PET imaging including 68Ga (Hoffend J. et al., Nucl Med Biol. 2005; 32(3): 287-292), 62Cu (Okazawa H. et al., J Nucl Med 1996; 37(7): 1080-1085), and 64Cu (Anderson C J et al., Nucl Med Biol. 1993; 20(4): 461-467). Compared with these radiometals, 18F has the advantages of being a pure positron emitter and having an ideal half-life. It is the dominant radioisotope used for PET imaging for both clinical applications and preclinical investigations.
As a protein, albumin can be labeled with 18F through reaction of N-succinimidyl 4-18F-fluorobenzoate (SFB) with an amine group or N-[2-(4-18F-fluorobenzamido)ethyl]maleimide (FBEM) on the thiol group. In one study, Wangler et al. prepared 4-(di-tert-butyl-18F-fluorosilyl)benzenethiol (18F-SiFASH) and coupled it directly to rat serum albumin (RSA) (Wangler B. et al., Bioconjug Chem 2009; 20(2): 317-321). However, high liver uptake was observed on the 18F-SiFA-RSA blood pool scan, indicating that the albumin structure may have been disrupted to some extent during labeling. One alternative is in vivo labeling of endogenous albumin with a pre-labeled albumin binder. Ideally, the binder will not affect the in vive behavior of the serum albumin such as circulation, extravascularization, and turn-over; thus the imaging results will reflect the distribution and metabolism of serum albumin accurately. Currently available albumin binders include small molecules, peptides that possess an albumin binding domain, and antibodies.
Identification of liver lesions is of critical importance due to the increasing incidence of primary hepatic malignancies worldwide and an increase in detection of benign liver lesions by the widespread use of abdomen cross-sectional imaging modalities. Although many typical lesions can be detected by traditional imaging tests such as ultrasound, CT, and MRI, there remains a challenge to diagnose atypical lesions. For example, hypervascular neuroendocrine tumors often share the same appearance as hemangiomas on MRI. Some atypical hepatic cysts may also show overlapping features with hepatic metastasis from ovarian malignancies.
The lymphatic system plays a key role in maintaining tissue interstitial pressure by collecting protein-rich fluid that is extracted from capillaries. The lymphatic system is also a critical component of the immune system. Many types of malignant tumors such as breast cancer, melanoma, and prostate cancer are prone to metastasize to regional lymph nodes (LNs), possibly through tumor associated lymphatic channels. The status of these sentinel LNs (SLNs) not only provides a marker for tumor staging but also serves as an indicator of prognosis. Consequently, detection and mapping of SLNs is a key step in therapeutic decision-making (Veronesi U, et al., Lancet 1997, 349(9069): 1864-1867).
One common method used in the clinic is a two-step procedure which consists of local administration of radionuclide-labeled colloids, mostly with technetium-99m, several hours before the injection of a vital dye such as Patent blue (isosulfan blue). SLNs can be visualized either by gamma scintigraphy or SPECT (single photon emission computed tomography). The SLNs during surgery can be located with a hand-held gamma ray counter and visual contrast of the blue dye. However, this method requires separate administration of two agents because of different rates of local migration of the colloidal particles and blue dye molecules. Due to the relatively low sensitivity and poor spatial resolution of scintigraphy and SPECT, it is highly desirable to develop new imaging probes for other imaging modalities. The objective is to improve the detection of SLNs either for noninvasive visualization or intrasurgical guidance.
Recently, imaging guided surgery, especially with fluorescent probes, has been intensively studied due to its low cost, simplicity, and adaptability. The limited tissue penetration of light is less critical because of open field of view during surgery. For example, NIR fluorescence dyes, such as indocyanine green (ICG), have been investigated for sentinel node navigation during surgery either alone or in combination with nanoformulations (Hirano A, et al., Ann Surg Oncol 2012, 19(13):4112-4116; Koo J, et al., Phys Med Biol 2012, 57(23):7853-7862). Owing to the nanometer-scale size, stability and strong fluorescence, various nanoparticles and nanoformulations have been applied for SLN imaging and showed promising results in preclinical models. However, most of these probes are composed of heavy metals making their clinical translation difficult due to the acute and chronic toxicity. In addition, scattering and tissue attenuation cause poor results for pre-surgical evaluation of SLNs using optical imaging.
Thus, there remains a need in the art for improved methods for imaging of blood pools and the lymphatic system.